In the past, the commercial preparation of chlorofluorocarbons (CFC's) using Lewis acid catalysts has been used to carry out a variety of fluorine-halogen exchange and hydrofluorination reactions. These catalysts have now been successfully adapted to perform similar reactions for the "Third Generation" hydrochlorofluorocarbons (HCFC's) and hydrofluorocarbons (HFC's).
These HFC's and HCFC's are generally prepared from HCC's and hydrogen fluoride (HF) by moderate halogen exchange in which the carbon to chlorine bonds of the HCC are broken and carbon to fluorine bonds are formed in their place. The metal of the Lewis acid acts in its catalytic capacity leading to a more productive exchange process requiring milder reaction conditions. Exemplifying such reactions is the synthesis of 1,1-difluoroethane (HFC-152a) which may be prepared by allowing 1,1-dichloroethane and HF to react in the presence of various Lewis acids, such as the halides of various metals, e.g., tin (IV), titanium (IV), antimony (III), antimony (V) and the like.
By-products formed in the preparation of the Lewis acid catalyzed HFC's include an assortment of oligomeric and polymeric materials and low molecular weight halogenated dimers. Many of these byproducts take the form of oils or tars. These byproducts are detrimental to the halogen exchange process because of the tar formation associated with the presence of oligomers and high molecular weight materials forms complexes with the catalyst, suppressing catalytic activity.
Prior art catalysts, such as the antimony (V) halides, used in the production of such halocarbons as fluorotrichloromethane, difluorodichloromethane and fluorodichloromethane have been kept active by maintaining a constant feed of the oxidizer, chlorine gas. In such cases, the accumulation of tars is very low. However, the tars do accumulate and eventually deactivate the catalyst. This deactivated (spent) catalyst must be removed, with regeneration being carried out at an off-site facility. One such regeneration is demonstrated in U.S. Pat. No. 4,722,774 and is accomplished by an aqueous acid salvation of the antimony halides to separate them from the tars, followed by an intensive process to dehydrate the recovered catalyst.
U.S. Pat. No. 3,806,589 discloses that spent antimony halide catalysts resulting from the fluorination of chlorinated hydrocarbons can be regenerated by first dissolving the spent catalyst in water. After a series of steps including precipitation of antimony oxide with ammonia, treatment of the precipitate with hydrochloric acid, reduction in the presence of a redox catalyst, reprecipitation with ammonia and again dissolving the precipitate in hydrochloric acid, the latter solution is distilled to obtain antimony (III) chloride, which may be used as fresh catalyst.
The recovery of spent antimony (V) chloride which had been used in the fluorination of chloro-substituted methanes is disclosed in U.S. Pat. No. 3,760,059. The catalyst is reduced to antimony trichloride in a trichloroethylene solution. Antimony (III) chloride precipitates and is converted to antimony (V) chloride. Commercial utility of this process appears remote, since heavy loadings of tar defeat this technique.
EP 0 798 043 discloses that fluorination catalysts used in the preparation of hydrohalogenoalkanes can be regenerated by treatment with chlorine and hydrogen fluoride at 250.degree. to 450.degree. C. However, the application of chlorine oxidation in a "de-tarring" step in the liquid phase is impractical, because the temperatures that are required to maintain combustion generate extremely high pressures in a closed system.